1. Field of the Invention
The present invention relates generally to cell reselection and, more particularly, to a cell reselection method using a base station's distance information, and to a mobile communication terminal using the cell reselection method.
2. Description of the Related Art
Generally, mobile communication terminals communicate with a mobile communication network through radio cells. Movement of a mobile communication terminal between cells may result in cell switching. Cell switching occurs in, for example, a handover procedure or a cell reselection procedure. Movement of a mobile communication terminal, while in an on-going call, causes the network to perform a corresponding handover procedure for sustenance of the call. A mobile communication terminal, while in an idle mode, may perform a cell reselection procedure to find a better serving cell.
Referring to FIG. 1, the cell reselection procedure is described below.
A mobile communication terminal begins the procedure by calculating C1 path loss parameters (path loss criteria) of a serving cell (or a current cell) and neighboring cells at regular intervals at step S105. Thereto the mobile communication terminal periodically measures the power of a broadcast channel signal of each cell. For example, in a Global System for Mobile Communications (GSM) system or General Packet Radio Services (GPRS) system, a mobile communication terminal repeatedly measures at least every 5 seconds the power of a signal on a broadcast control channel (BCCH) of each of a serving cell and neighboring cells in order to calculate corresponding C1 path loss parameters.
The C1 path loss parameter of a cell is calculated using the following Equation 1:C1=RXLEV−RXLEV_ACCESS_MIN−Max(0,MS—TXPWR—CCH−P)  (1)where RXLEV denotes a received signal level of the BCCH signal of the cell, RXLEV_ACCESS_MIN denotes the minimum received signal level for access to the cell, MS_TXPWR_CCH denotes the maximum transmitting power that the mobile communication terminal is permitted to use for access to the cell, and P denotes the maximum transmitting power of the mobile communication terminal.
RXLEV is a value measured by the mobile communication terminal, RXLEV_ACCESS_MIN and MS_TXPWR_CCH are values received from a corresponding base station, and P is a value specific to the mobile communication terminal.
After calculation of the C1 path loss parameters, the mobile communication terminal determines whether the C1 path loss parameter of the serving cell is less than zero at step S110. If the C1 path loss parameter of the serving cell is less than zero, the mobile communication terminal performs a cell reselection operation at step S115. It is considered impossible for a mobile communication terminal to receive a communication service from a cell whose C1 path loss parameter is less than zero, owing to a large amount of path loss in the cell.
If at least one suitable cell that can provide a communication service is found through the cell reselection operation at step S120, the mobile communication terminal selects one of the found suitable cells as a new serving cell at step S125, and receives a communication service from the new serving cell in step S130.
If no suitable cell is found, the mobile communication terminal enters into a no-service mode at step S135.
In such a conventional cell reselection procedure, the suitability of a serving cell for a communication service is determined on the basis of a corresponding C1 path loss parameter that is calculated under the control of a mobile communication terminal. However, different mobile communication terminals that are receiving services from the same serving cell may obtain different results in determination of the service suitability of the serving cell. That is, when mobile communication terminals that are different, for example in design model or manufacturer, are located at an identical service area where an electric field is weak, some of the mobile communication terminals may determine the serving cell to be suitable and others may determine the serving cell to be unsuitable.
This is because a C1 path loss parameter of the serving cell calculated by a mobile communication terminal may be different from that calculated by another mobile communication terminal. Different mobile communication terminals may have different degrees of precision in their components and different internal processing algorithms according to design models or manufacturers. Hence, although mobile communication terminals are located within an identical service area, they may obtain different results in determination of the service suitability of the serving cell. That is, even when a serving cell is in reality in a suitable state, a mobile communication terminal may determine that the serving cell is unsuitable for a communication service.